You just need to set I18n.locale in a `before_action`.

Add the following line to `config/initializers/active_admin.rb`:

```ruby
#config/initializers/active_admin.rb
config.before_action :set_admin_locale
```
or if you are using Rails 5.1+:

```ruby
#config/initializers/active_admin.rb
config.before_action :set_admin_locale
```
and define the following method in your ```application_controller.rb```:

```ruby
def set_admin_locale
  I18n.locale = :en
end
```

Of course, put whatever logic you want into the `set_admin_locale` method. If your `AdminUser` class has a `locale` field to store the user's locale, you can do:

```ruby
def set_admin_locale
  I18n.locale = current_admin_user && current_admin_user.locale || I18n.default_locale
end
```